The clay mèlanges of the south-ferghana type (Tien Shan): data review and geological model of development

Tectonic mixtites, or mélanges, with clay cement are specific to the western part of the Paleozoic Southern Tien Shan in the Turkestan‒Alai mountain system. The South-Fergana type mélanges, or Tul`‒Sartalian, have a sandy-clayey matrix of Silurian (less often ‒ Ordovician) age and block inclusions of sedimentary, igneous and partially metamorphic rocks with an age from the late Precambrian to the middle Carboniferous. The mélanges were formed as a result of the destruction of water-saturated sandy-clayey deposits, which occurred in the upper part of the subduction channel, on the southern active margin of the Kazakhstan paleocontinent. At the beginning of the collisional stage at the turn of the Early and Middle Carboniferous, they were brought to the surface and became part of the accretionary prism, which also included ophiolite sheets. In the time interval the Moscovian‒the beginning of Permian, the mélanges, together with other accreted formations, were involved in a series of collisional thrust sheets and moved to the south. The reverse northern thrust of tectonic plates with mélanges, observed within the Southern Fergana, arose as a result of the general collision at the beginning of the Permian between the Kazakhstan and Karakum‒Tajik paleocontinents, with subsequent orogenic collapse and granite magmatism. The consequence of this was the disintegrated position of individual fields and structural units with mélanges. The South-Fergana type clay mélanges contain block inclusions in island arcs and marginal basins that formed in the early Turkestan paleo-ocean during the terminal Precambrian and the Early Paleozoic.